The ongoing NHLBI-funded Occluded Artery Trial (OAT) is designed to test the hypothesis that opening an occluded infarct related artery (IRA) 3-28 days following an MI will reduce the composite endpoint of mortality, recurrent MI, and NYHA class IV heart failure over three-year follow-up. OAT has currently randomized approximately 1100 patients in 240 centers in 24 countries. Among the mechanisms proposed to explain benefit of late revascularization, recovery of LV function and attenuation of LV remodeling due to restoration of blood flow to viable myocardium is the most plausible. We propose an ancillary study of OAT to assess the effect of myocardial viability on LV remodeling post-MI, and to explore the relationships between retained viability of the infarct zone, LV remodeling, and the response to the OAT intervention, late PCI of the IRA. To this end, we propose to enroll 200 patients in the Viability and Remodeling in OAT Ancillary Study (OAT-NUC). The primary specific aims of this study are 1) to test the hypothesis that patients who demonstrate preservation of viability within the infarct zone will evidence less progressive remodeling compared to patients exhibiting predominant infarct, and 2) to test the hypothesis that preservation of viability will modify the treatment effect of randomization to late revascularization of post-MI patients with an occluded IRA 3-28 days after an AMI. All patients will have resting gated Tc-99m sestamibi SPECT imaging at baseline, and again 1 year later. Parameters of baseline viability within the infarct zone, and serial measures of LV volume change and function will be centrally assessed by the Cardiac Imaging Core Laboratory at Tufts-New England Medical Center, a group with substantial experience analyzing cardiac imaging data in multicenter clinical trials. The major study endpoint to address the hypotheses will be serial change in LV end-diastolic volume, based on the degree of preservation of viability within the defined infarct zone. Sample size calculations are based on data evaluated by the same group from a similar number of patients studied at a similar number of clinical sites. Hence, sample size projections are robust and appropriate for the current trial, and the investigative group has the experience to carry out the proposed study with high quality. OAT-NUC is the only viability ancillary study of OAT, and thus serves a critical need in assessing potential mechanisms of the findings in OAT.